Side rails? As in bumper-bowling?
Or the kick-back boards on both walls of the pin deck?
These are wood, and either within USBC specifications in a sanctioned facility, or not. Last time I checked these are quite hard. They have to be hard or the pins will not "bounce" off them. If you ever find some
soft ones let me know because the scoring pace would be very low, and I could compete and dominate against 600 revs...
P.S. To ballreviews regulars, sorry my sarcasm MADE me do it.